[0001] The invention relates to a process for the preparation of a phenol, by oxidative
decarboxylation of the corresponding benzoic acid in the presence of a copper-containing
catalyst, wherein accumulation of tar components in the reactor is avoided by continuously
removing a portion of the reaction mixture from the reactor and subjecting the removed
portion to an extraction, by contacting the removed portion with water and an auxiliary
liquid, which auxiliary liquid is not miscible with water, wherein after extraction
an aqueous liquid is obtained with benzoic acid and copper salts and an organic solution
with tar components, wherein the aqueous solution is recycled to the reactor.
[0002] The preparation of a phenol by an oxidative decarboxylation of the corresponding
benzoic acid has been known for a long time already. Such a process is already described
in NL-B-90684. According to NL-B-90684, the oxidation, the decarboxylation as well
as the hydrolysis are carried out in the liquid phase, in a single process step, at
a temperature of at least 200°C, preferably 230-250°C.
[0003] In the said process many by-products are formed, mainly in the form of tar components.
[0004] Accumulation of these tar components in the reactor can be prevented by periodically
or continuously withdrawing a portion of the liquid reaction mixture from the reactor
and then effecting a separation between the tar components and the substances which
are still valuable in the process, such as benzoic acid and copper salts and - if
present - magnesium salts. The substances which are valuable in the process are subsequently
returned to the reactor. The remaining solution containing the tar components will
in general be burnt.
[0005] This separation of the tar components and substances which are valuable in the process
is usually effected by extraction with an extraction agent which is a solvent for
benzoic acid and copper and magnesium salts.
[0006] In general, water is used as extraction agent for this so-called tar extraction.
[0007] In practice it appears that such a separation by water extraction does not give satisfactory
results because, among other things, rather much copper remains behind in the solution
containing the tar components. Moreover, the ratio of the extraction selectivity toward
the substances of value in the process and the extraction selectivity toward the tar
components is unfavourable. In practice this means that if for instance a multistage
countercurrent extraction is applied in order to achieve a certain recovery efficiency
for these valuable substances, still a relatively large quantity of tar components
is returned to the reactor with the aqueous extraction agent.
[0008] Extraction selectivity is the percentage of the quantity of a substance originally
present in the reaction mixture to be extracted, which the extraction agent contains
after one extraction step. The recovery efficiency is the percentage of the quantity
of a substance originally present in the reaction mixture to be extracted which the
extraction agent contains after completion of the extraction, which may comprise one
or more extraction steps.
[0009] US-A-3130235 describes a process wherein the reaction mixture to be separated is
extracted using an extraction agent and an auxiliary liquid which is immiscible with
the extraction agent. The auxiliarly liquid is chosen in such a way that the density
of the auxiliarly liquid containing the tar particles is higher than the density of
the extraction agent containing the desirable components, in order that any suspended
tar particles that tend to settle, will accumulate at the bottom of the vessel. In
this way, the extraction selectivity toward benzoic acid and copper and magnesium
salts is improved. It is not mentioned whether the extraction selectivity toward the
tar components is (favourably) reduced. By preference, the lower halogenated aliphatic
hydrocarbons, in particular perchloroethene, are used as auxiliary liquid. The extraction
agent is preferably water with a water-miscible alcohol, in particular methanol.
[0010] A drawback of this known method is the use of substances foreign to the process,
such as the halogenated aliphatic hydrocarbon and the alcohol. This requires additional
purification steps in the process. Another drawback is that the organic solution containing
the auxiliarly liquid, will contain halogenated hydrocarbons after the extraction,
so that further processing of this solution without far-reaching environmental measures
is virtually impossible.
[0011] The object of the invention is a simple separation by means of an extraction, using
water as extraction agent, resulting in sufficient separation of benzoic acid, copper
and any magnesium salts from the tar components.
[0012] This object is achieved in that the extraction is performed with 0.2 to 14 volume
parts of auxiliary liquid per volume part of removed reaction mixture and with 0.5
to 10 volume parts of water per volume part of removed reaction mixture, the quantity
of auxiliary liquid being so chosen that the resulting organic solution, containing
the auxiliary liquid, has a lower density than the aqueous solution and that the temperature
during the extraction is higher than 80°C.
[0013] When the extraction is performed in this way, it appears that the water extracts
both benzoic acid and the copper and magnesium salts very selectively. The tar components
remain almost completely behind in the organic solution containing the auxiliary liquid
and the extracted reaction mixture. This process is highly advantageous because it
gives a good extraction selectivity toward the useful components without the use of
process foreign or process hindering substances and without formation of solids which
make additional process steps necessary.
[0014] The auxiliary liquids according to the invention are for instance non-halogenated
hydrocarbons. The hydrocarbons in general for instance are non-substituted hydrocarbons
having 5 to 10 carbon atoms. Examples of suitable auxiliary liquids are benzene, octane
and toluene. These substances will not present special environmental problems when
they are used in a process according to the invention. Particularly suitable are toluene
and benzene, because these substances are non-process-foreign substances. Toluene
is no process foreign substance in the process described in NL-B-90684, because the
benzoic acid is prepared by oxidation of toluene. As a consequence, in an existing
process there are in general provisions for the processing of toluene and the purification
of toluene-containing flows. Further, in the distillative purification of phenol,
toluene is in general used as auxiliary liquid substance in the separation of phenol
and water. Benzene is not a process foreign substance because it is obtained as a
by-product in the oxidative decarboxylation of benzoic acid.
[0015] Further, it has appeared that the process according to the invention results in a
very efficient separation of a variety of tar components. Because a smaller amount
of tar components is returned to the reactor at the same level of benzoic acid recovery,
tar formation in the reactor is reduced.
[0016] It is a special advantage of the present invention that the copper concentration
in the reaction zone can be increased, without causing an increase in tar formation
compared with the state of the art. An increased copper concentration means a larger
production of phenol per unit of reactorvolume. In this way the capacity and selectivity
for phenol production of an existing plant can be increased.
[0017] In US-A-3130205, mentioned above, toluene and benzene are mentioned as possible auxiliary
liquids for improvement of the extraction. In the same patent specification, however,
the use of toluene and benzene is advised against. According to US-A-3130205, because
of the low density of benzene and toluene, tar particles suspended in the auxiliarly
liquid will collect at the liquid interphase, thus complicating the problem of making
a clean separation of the two liquid phases. Another reason for advising against the
use of benzene and toluene is the greater tendency of these solvents to dissolve some
of the desirable components of the reaction mixture, which results in an unfavourable
loss of desirable reaction products and an inefficient extraction procedure.
[0018] In a process according to the invention the use of an auxiliary liquid appears to
be advantageous owing to the employment of a countercurrent extraction column. A series
of mixer-settlers will also give the desired results. The use of an auxiliary liquid
offers the further advantage that the viscosity of the tar-component-containing organic
solution is lowered, which makes it easier to treat this liquid. Another advantage
is that after the extraction the aqueous solution and the organic solution de-mix
easier.
[0019] GB-B-1297212 also describes a process in which a tar-component-containing reaction
mixture in the phenol preparation process is extracted with water (acidified, if required)
in the presence of toluene.
[0020] According to GB-B-1297212 the extraction with water is carried out in such a way
that only the copper and magnesium salts are extracted and that the benzoic acid remains
behind in the organic solution. This is achieved either by extracting with a lot of
water, in which case dissolution of benzoic acid in the aqueous solution is prevented
by raising the degree of acidity of the aqueous solution, or by extracting with a
small quantity of water, so that the metal salts that are present are fully precipitated
and only little benzoic acid can dissolve. The benzoic acid is then recovered by distilling
it out of the tar-component-containing organic solution. Next, the metal salts and
the benzoic acid are returned to the reactor. In this way, it is said, in particular
less copper remains behind in the organic solution, so that less copper catalyst is
lost. Drawbacks of this process are the large number of process steps, including the
invariably difficult solids handling steps, and the use of an acidified extraction
agent.
[0021] JP-A-59029626 also describes a process in which toluene is supplied to a tar-component-containing
reaction mixture, in the phenol production process, before this mixture is extracted
with water. In that process the extraction with water is effected with the molar ratio
between water and the total amount of catalytically active components (e.g. Cu and
Mg) lying between 10:1 and 100:1, at a temperature between 20 and 180°C and a pressure
between 0.1 and 1 MPa. Characteristic of that invention is that the catalytically
active components precipitate in the aqueous solution. A drawback of the process is
that for further treatment of this solid phase the difficult process steps such as
filtration and crystallization are required. Another drawback of this known process
is that after the extraction so much benzoic acid still remains behind in the organic
solution that an additional process step is required to remove the benzoic acid and
return it to the reactor. In the process according to the present invention this additional
process step is not required.
[0022] The preparation of phenol by means of an oxidative decarboxylation of benzoic acid
is carried out with a copper-containing catalyst. The copper concentration in the
(separated) reaction mixture will in general be between 0.5 and 10 wt.% relative to
the total mixture. By preference, the copper concentration will be between 1 and 5
wt.%. Besides copper the catalyst will in general contain a co-catalyst. Such a co-catalyst
is preferably an element from groups V, VI, VII or VIII, or from the group of lanthanides
and actinides, of the Periodic System of the Elements. In addition, promotors can
be used; in particular (earth) alkaline metals, such as magnesium or lithium, are
suitable as such. By preference, the concentration of these co-catalysts and/or these
promotors is between 1 and 10 wt.%.
[0023] The composition of the reaction mixture that is withdrawn from the phenol preparation
reactor depends on, among other things, the catalyst system used, the concentrations
of the catalyst and the starting materials, and the process conditions. The phenol
concentration in the separated reaction mixture can vary from almost nil to 6 wt.%.
[0024] The amount of tar components in the separated reaction mixture will in general be
between 1 and 30 wt.%, in particular between 4 and 30 wt.%.
[0025] Besides phenol and tar components a quantity of diphenyl ether and phenyl benzoate
is formed, in general amounting to a total of between 1 and 5 wt.%. The quantity of
non-converted benzoic acid in the form of free benzoic acid and metal benzoates constitutes
the remaining portion of the mixture. The metal benzoates are benzoates of the copper
and the co-catalyst metals mentioned above.
[0026] The pressure and temperature at which the extraction is effected are in general chosen
such that the water/auxiliary liquid/reaction mixture system does not boil. In general
the pressure is between 0.1 and 6 MPa and the temperature between 80 and 250°C. Preferably,
the pressure is between 0.2 and 4 MPa and the temperature between 100 and 220°C, in
particular between 120 and 160°C. An additional advantage of working at a temperature
higher than 80°C is that no substantial additional cooling is required for the reaction
mixture leaving the reactor. The temperature decrease can be achieved simply by effecting
the extraction with auxiliary liquid and/or water at room temperature.
[0027] The volume ratio between auxiliary liquid and the removed reaction mixture is in
general between 0.2 and 10; preferably, it will be between 0.5 and 5 because a quantity
of auxiliary liquid, in particular a quantity of toluene, larger than 5 times the
volume of the removed reaction mixture is economically less attractive. More in particular,
a ratio of about 1 appears to give very good results. The volume ratio between water
and removed reaction mixture is in general between 0.5 and 10, preferably between
0.8 and 2.5, in particular around 1.5.
[0028] The extraction is preferably carried out in several steps, with the organic phase
and the water phase in countercurrent. Such a process could be carried out for instance
in a series of mixer-settlers. In this case it would be possible, instead of the above-mentioned
countercurrent principle, to use clean water in each separation. The extraction can
also be carried out well in an extraction column. Preferably, such an extraction column
will be a rotating disk column or a column of similar type. The required number of
extraction steps can easily be determined by one skilled in the art and will as a
rule be between 4 and 12.
[0029] The removed reaction mixture, the auxiliary liquid and the water can be contacted
with each other simultaneously. It is also possible first to contact the reaction
mixture with the auxiliarly liquid and subsequently with the water.
[0030] The auxiliary liquid in the organic solution will in general be recovered after the
extraction. The auxiliary liquid according to the invention can be separated from
the organic solution in a simple manner, for instance by means of distillation. If
desired, the auxiliary liquid can be re-used in the process according to the invention.
Any useful components which are separated from the organic solution together with
the auxiliary liquid are returned to the process in this way. The organic phase will
in general be burnt after removal of the auxiliary liquid.
[0031] The invention will be elucidated further in the following examples, without being
restricted thereto.
Example I
[0032] An autoclave was filled with 560 ml toluene, 480 ml water and 380 ml reaction mixture
of the following composition:
phenol: |
1.0 wt.% |
total benzoic acid/benzoate: |
67.8 wt.% |
magnesium: |
3.5 wt.% |
copper: |
1.0 wt.% |
tar components: |
25.0 wt.% |
water: |
1.7 wt.% |
At a temperature of 140°C and a pressure of 1 MPa, stirring was applied for 15 minutes
at 1470 rpm. The stirrer was stopped and after 6 minutes it appeared that two separate
phases had formed, the upper phase being the organic solution and the lower phase
the aqueous solution. The total benzoate content was determined by HPLC (High Pressure
Liquid Chromatography) analysis. The compositions of the upper and the lower phase
were:
|
organic solution (wt.%) |
aqueous solution (wt.%) |
phenol: |
0.4 |
0.2 |
total benzoic acid/benzoate: |
10.8 |
29.0 |
magnesium: |
0.1 |
2.0 |
copper: |
0.1 |
0.4 |
tar components: |
13 |
1 |
toluene: |
64 |
1 |
water: |
11 |
67 |
The extraction selectivity toward benzoate was 72%, toward the tar components 7%,
toward copper 75% and toward magnesium 95%.
Example II
[0033] Example I was repeated. The organic solution was separated from the aqueous solution.
Next, 480 ml 'clean' water was added to the organic solution. Under the same conditions
as in Example I the mixture was mixed and analyzed. The extraction efficiency after
these two extraction steps was 82% for benzoate, 8% for the tar components, 87% for
copper and 100% for magnesium.
Experiment A
[0034] An autoclave was filled with 600 ml water and 500 ml reaction mixture of the same
composition as the reaction mixture used in Example I. At a temperature of 140°C and
a pressure of 1 MPa, stirring was applied for 15 minutes at 1470 rpm. The stirrer
was stopped and after 15 minutes it appeared that two separate phases had formed,
the upper phase being the aqueous solution and the lower phase the organic solution.
The total benzoate content was determined by HPLC analysis. The compositions of the
upper and the lower phase were:
|
organic solution (wt.%) |
aqueous solution (wt.%) |
phenol: |
0.6 |
0.4 |
total benzoic acid/benzoate: |
25.6 |
30.6 |
magnesium: |
0.3 |
1.6 |
copper: |
1.3 |
0.4 |
tar components: |
66 |
9 |
water: |
6 |
58 |
The extraction selectivity toward benzoate was 94%, toward the tar components 74%,
toward copper 82% and toward magnesium 99%.
1. Process for the preparation of a phenol, by oxidative decarboxylation of the corresponding
benzoic acid in the presence of a copper-containing catalyst, wherein accumulation
of tar components in the reactor is avoided by continuously removing a portion of
the reaction mixture from the reactor and subjecting the removed portion to an extraction,
by contacting the removed portion with water and an auxiliary liquid, which auxiliary
liquid is not miscible with water, wherein after extraction an aqueous liquid is obtained
with benzoic acid and copper salts and an organic solution with tar components, wherein
the aqueous solution is recycled to the reactor, characterized in that the extraction
is performed with 0.2 to 14 volume parts of auxiliary liquid per volume part of removed
reaction mixture and with 0.5 to 10 volume parts of water per volume part of removed
reaction mixture, the quantity of auxiliary liquid being so chosen that the resulting
organic solution, containing auxiliary liquid, has a lower density than the aqueous
solution and that the temperature during the extraction is higher than 80°C.
2. Process according to claim 1, characterized in that the auxiliary liquid is a non-halogenated
hydrocarbon compound.
3. Process according to claim 2, characterized in that the hydrocarbon compound is toluene.
4. Process according to any one of claims 1-3, characterized in that the removed reaction
mixture is mixed with 0.5 to 5 volume parts of auxiliary liquid per volume part of
removed reaction mixture.
5. Process according to any one of claims 1-4, characterized in that 0.8 to 2.5 volume
parts of water per volume part of removed reaction mixture are used for the extraction.
6. Process according to any one of claims 1-5, characterized in that the extraction is
carried out in two or more extraction steps.
7. Process according to any one of claims 1-6, characterized in that the auxiliary liquid
is recovered from the organic solution after the extraction and returned to the process
and that the remaining solution with tar components is drawn off.
8. Process according to any one of claims 1-7, characterized in that the extraction is
carried out at a pressure between 0.2 and 4 MPa and a temperature between 100 and
220°C, pressure and temperature being chosen such that the water/auxiliarly liquid/reaction
mixture does not boil.
9. Process according to any one of claims 1-8, characterized in that the removed portion
of reaction mixture is first contacted with the auxiliary liquid and subsequently
with water.
10. Process as substantially described in the specification and the examples.